A colonial tunicate (sea squirt) that has its origins in Japan, China and southern Siberia, and introduced to Pacific Northwest waters, Botrylloides violaceus is a colonial tunicate consisting of many small individuals called zooids. Each zooid pumps water through its own siphon, filters out oxygen and feeds on small organisms such as phytoplankton, zooplankton, oyster and mussel larvae, and other suspended organic materials. It then discharges the filtered water and waste into a common space that is shared within a system of other zooids. The arrangement of a colony is that of several systems within a clear, firm, fleshy matrix, each system consisting of dozens of zooids seen on the surface as elongated ovals or meandering, occasionally branching double rows or chains. The filtered water exits through a shared pore between these rows. The zooids with a colony are the same, single color. The color of a complete colony, however, can range from tan to yellowish orange, to brown or lavender.

B. violaceus prefers protected marine waters and can reproduce rapidly, fouling ship hulls, docks, piers and shellfish aquacultures. It can be found on any substrate, including gravel, seabed, metal, plastic, rope, fiberglass, wood, and shellfish. It does not appear to be as aggressive as Didemnum vexillum, forming generally smaller colonies, but can still significantly out compete other organisms for space and food.

Unlike the solitary tunicates, the colonial B. violaceus broods its larvae internally by the fertilization of its eggs from male gametes of nearby colonies. It then releases the planktonic larvae into the water column where it eventually settles on a firm surface where it metamorphoses into a zooid and then buds off to the sides asexually to form the multiple systems that make up a colony. The larvae are very short lived, likely settling onto a firm surface and near the point of origin. However, fragments from a colony can survive indefinitely, reproducing asexually while drifting, or traveling within ship ballast water to colonize new areas.